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Hard carbon is the most common anode material for Na-ion battery. The structure of the hard carbon and the dynamics of Na-ion in hard carbon were studied with small- and wide-angle neutron scattering and muon spin relaxation technique. The neutron scattering revealed the increase of interlayer distance between graphenes and decrease of the size of nanopores with increasing sodium intercalation in hard carbon. The muon spin relaxation revealed that a systematic increase in the field fluctuation rate with increasing temperature evidenced a thermally activated sodium diffusion. Assuming the two-dimensional diffusion of Na-ion in the graphene layers, the self-diffusion coefficient of Na-ion was estimated to be 2.6 × 10−11 cm2/s at 310 K, with a thermal activation energy of 39(7) meV.

Sodium transition metal oxides with a layered structure are one of the most widely studied cathode materials for Na+-ion batteries. Since the mobility of Na+ in such cathode materials is a key factor that governs the performance of material, electrochemical and muon spin rotation and relaxation techniques are here used to reveal the Na+-ion mobility in a P2-type Na0.5MgxNi0.17-xMn0.83O2 (x = 0, 0.02, 0.05 and 0.07) cathode material. Combining electrochemical techniques such as galvanostatic cycling, cyclic voltammetry, and the galvanostatic intermittent titration technique with mu+SR, we have successfully extracted both self-diffusion and chemical-diffusion under a potential gradient, which are essential to understand the electrode material from an atomic-scale viewpoint. The results indicate that a small amount of Mg substitution has strong effects on the cycling performance and the Na+ mobility. Amongst the tested cathode systems, it was found that the composition with a Mg content of x = 0.02 resulted in the best cycling stability and highest Na+ mobility based on electrochemical and mu+SR results. The current study clearly shows that for developing a new generation of sustainable energy-storage devices, it is crucial to study and understand both the structure as well as dynamics of ions in the material on an atomic level. Title in dissertation list of papers: Na-mobility in P2-type Na0.5MgxNi0.17−xMn0.83O2 (0 ≤ x ≤ 0.07) from electrochemical and muon-spin relaxation studies

Observation of a quantum spin liquid (QSL) state is one of the most important goals in condensed-matter physics, as well as the development of new spintronic devices that support next-generation industries. The QSL in two dimensional quantum spin systems is expected to be due to geometrical magnetic frustration, and thus a kagome-based lattice is the most probable playground for QSL. Here, we report the first experimental results of the QSL state on a square-kagome quantum antiferromagnet, KCu6AlBiO4(SO4)5Cl. Comprehensive experimental studies via magnetic susceptibility, magnetisation, heat capacity, muon spin relaxation (μSR), and inelastic neutron scattering (INS) measurements reveal the formation of a gapless QSL at very low temperatures close to the ground state. The QSL behavior cannot be explained fully by a frustrated Heisenberg model with nearest-neighbor exchange interactions, providing a theoretical challenge to unveil the nature of the QSL state., There exist many theoretical models of frustrated quantum magnetism, most of which lack convincing experimental realisations. Here the authors combine several experimental methods to argue that KCu6AlBiO4(SO4)5Cl behaves as a square-kagome-lattice quantum Heisenberg antiferromagnet.

The long charge carrier lifetime of the hybrid organic-inorganic perovskites (HOIPs) is the key for their remarkable performance as a solar cell material. The microscopic mechanism for the long lifetime is still in debate. Here, by using muon spin relaxation technique that probes the fluctuation of local magnetic fields, we show that the muon depolarization rate ($\Delta$) of a prototype HOIP methylammonium lead iodide (MAPbI$_3$) shows a sharp decrease with increasing temperature in two steps above 120 K and 190 K across the structural transition from orthorhombic to tetragonal structure at 162 K. Our analysis shows that the reduction of $\Delta$ is quantitatively in agreement with the expected behavior due to the rapid development of MA jumping rotation around the $C_3$ and $C_4$ symmetry axes. Our results provide direct evidence for the intimate relation between the rotation of the electric dipoles of MA molecules and the charge carrier lifetime in HOIPs., Comment: 7 pages, 4 figures

Magnesium hydride has great potential as a solid hydrogen (H) storage material because of its high H storage capacity of 7.6 wt%. However, its slow hydrogenation and dehydrogenation kinetics and the high temperature of 300 $^\circ$C required for decomposition are major obstacles to small-scale applications such as automobiles. The local electronic structure of interstitial H in MgH$_2$ is an important fundamental knowledge in solving this problem, which has been studied mainly based on density functional theory (DFT). However, few experimental studies have been performed to assess the results of DFT calculations. We have therefore introduced muon (Mu) as pseudo-H into MgH$_2$ and investigated the corresponding interstitial H states by analyzing their electronic and dynamical properties in detail. As a result, we observed multiple Mu states similar to those observed in wide-gap oxides, and found that their electronic states can be attributed to relaxed-excited states associated with donor/acceptor levels predicted by the recently proposed "ambipolarity model". This provides an indirect support for the DFT calculations on which the model is based via the donor/acceptor levels. An important implication of the muon results for improved hydrogen kinetics is that dehydrogenation, serving as a $reduction$ for hydrides, stabilizes the interstitial H$^-$ state., Comment: 14 pages, 9 figures

At the J-PARC muon facility (MUSE), new beamlines started operation recently. H-line is a high-intensity pulsed muon beamline for fundamental physics experiments. The first beam of the H-line was delivered to its first branch (H1 area) in January 2022, where a precise measurement of the muonium hyperfine structure and a search for μ-e conversion will be conducted. Further extension of the second branch of the H-line for a muon g-2/EDM experiment and a transmission muon microscope project is also ongoing. In addition, the second branch of the surface muon beamline (S2 area of the S-line) was opened for a muonium 1S-2S spectroscopy experiment in FY2021. In this paper, the recent upgrade and present status of the J-PARC muon facility and its prospects are presented.

Semimetallic osmium pyrochlore oxide Cd2Os2O7 undergoes a magnetic transition to an all-in–all-out (AIAO)-type order at 227 K, followed by a crossover to an AIAO insulator at around 210 K. Here, we studied the isostructural and isoelectronic compound Hg2Os2O7 through thermodynamic measurements, muon spin rotation (μSR) spectroscopy and neutron diffraction experiments. A similar magnetic transition, probably to an AIAO-type order, was observed at 88 K, while the resistivity showed a decrease at the transition and remained metallic down to 2 K. Thus, the ground state of Hg2Os2O7 is most likely an AIAO semimetal, which is analogous to the intermediate-temperature state of Cd2Os2O7. Hg2Os2O7 exists on the verge of the metal–insulator boundary on the metal side and provides an excellent platform for studying the electronic instability of 5d electrons with moderate electron correlations and strong spin–orbit interactions.

Significance Hybrid organic–inorganic perovskites (HOIPs) are among the most promising materials for next-generation solar cells that combine high efficiency and low cost. The record efficiency of HOIP-based solar cells has reached above 25%, and they can be manufactured using simple solution-processing methods that can be drastically cheaper than the current commercial solar cell technologies. Despite the progress so far, the microscopic mechanism for the high solar cell efficiency in HOIPs is yet to be understood. In this study, we show that the ability of organic molecules to rotate on an appropriate time scale in HOIPs can extend the lifetime of photoexcited charge carriers and lead to higher efficiency. This insight can guide the progress toward improved solar cell performance., The long charge carrier lifetime of the hybrid organic–inorganic perovskites (HOIPs) is the key for their remarkable performance as a solar cell material. The microscopic mechanism for the long lifetime is still in debate. Here, by using a muon spin relaxation technique that probes the fluctuation of local magnetic fields, we show that the muon depolarization rate (Δ) of a prototype HOIP methylammonium lead iodide (MAPbI3) shows a sharp decrease with increasing temperature in two steps above 120 K and 190 K across the structural transition from orthorhombic to tetragonal structure at 162 K. Our analysis shows that the reduction of Δ is quantitatively in agreement with the expected behavior due to the rapid development of methyl ammonium (MA) jumping rotation around the C3 and C4 symmetry axes. Our results provide direct evidence for the intimate relation between the rotation of the electric dipoles of MA molecules and the charge carrier lifetime in HOIPs.

Semimetallic osmium pyrochlore oxide Cd

We report the results of a muon spin relaxation study of slow magnetic fluctuations in the pseudogap phase of underdoped single-crystalline ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}, y=6.77$ and 6.83. The dependence of the dynamic muon spin relaxation rate on applied magnetic field yields the rms magnitude ${B}_{\mathrm{loc}}^{\mathrm{rms}}$ and correlation time ${\ensuremath{\tau}}_{c}$ of fluctuating local fields at muon sites. The observed relaxation rates do not decrease with decreasing temperature below the pseudogap onset at ${T}^{*}$, as would be expected for a conventional magnetic transition; both ${B}_{\mathrm{loc}}^{\mathrm{rms}}$ and ${\ensuremath{\tau}}_{c}$ are roughly constant between ${T}^{*}$ and the superconducting transition. NMR relaxation rates due to putative loop-current fluctuations are estimated and found to be too small to be observed. Our results put strong constraints on the theories of the anomalous pseudogap magnetism in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$.

We performed muon-spin-rotation/-relaxation measurements to investigate the magnetic behavior of ${T}^{\ensuremath{'}}$-type ${\mathrm{Eu}}_{2}{\mathrm{CuO}}_{4}$ (ECO), which is the parent compound of electron-doped cuprate superconductors, and the effects of oxygen-reduction annealing on its magnetism. In as-sintered (AS) ECO, we clarified the development of magnetic correlations upon cooling below ${T}_{\mathrm{N}1}$ (= 265 K) as well as the coexistence of a dominant fluctuating spin state and partially ordered spin state in the temperature range between $\ensuremath{\sim}150$ K and ${T}_{\mathrm{N}1}$. Upon further cooling, uniform long-range magnetic order was observed below ${T}_{\mathrm{N}2}=110$ K, which is close to the ordering temperature of 115 K in ${T}^{\ensuremath{'}}$-type ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ (LCO) [Phys. Rev. B 82, 180508(R) (2010)]. For oxygen-reduction-annealed ECO, a similar ordering sequence with the same ${T}_{\mathrm{N}2}$ was observed but without the partially ordered spin state. Therefore, the fluctuating spin state over a wide temperature range and a ${T}_{\mathrm{N}2}$ less than the N\'eel temperature (${T}_{\mathrm{N}2}\ensuremath{\approx}{T}_{\mathrm{N}1}$) in T-type LCO are common features of the ${T}^{\ensuremath{'}}$-type parent ${R}_{2}{\mathrm{CuO}}_{4}$ ($R\mathrm{CO}$, $R$: rare-earth ion). The origin of the partially ordered spin state in AS ECO is discussed from the viewpoint of chemical defect. Furthermore, we discuss the roles of electron doping and repairing defect in the observed effect of annealing on the magnetism of ${T}^{\ensuremath{'}}$-type $R\mathrm{CO}$.

Hidden magnetic order in the correlated iridate ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}{\mathrm{O}}_{4}, x=0.05$ and 0.1, has been studied using muon spin relaxation spectroscopy. In zero field (ZF) and weak longitudinal fields (LFs) ($\ensuremath{\lesssim}2\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$), the muon spin relaxation data indicate that static and dynamic local fields coexist at each muon site, and can be well described by exponentially damped static Lorentzian Kubo-Toyabe functions. The ZF relaxation rate is dominated by the static-field distribution, and a broad relaxation rate maximum at 175 K for $x=0.1$ in ZF is attributed to muon diffusion and trapping. For $\text{LF}\ensuremath{\gtrsim}2\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$ the static rate is completely decoupled, and the exponential decay is due to dynamic spin fluctuations. The temperature dependencies of the relaxation rates exhibit maxima at 215 K ($x=0.05$) and 175 K ($x=0.1$), in agreement with previous second harmonic generation and polarized neutron diffraction determinations of transition temperatures to a hidden-order state. The maxima are most likely due to critical slowing down of electronic spin fluctuations. The field dependencies of the dynamic spin fluctuation rates can be well described by the Redfield relation, from which the rms width ${B}_{\mathrm{loc}}^{\mathrm{rms}}$ and correlation time ${\ensuremath{\tau}}_{\mathrm{c}}$ of the fluctuating field are obtained. Values of ${\ensuremath{\tau}}_{c}$ are in the range of 1.5--4 ns for $x=0.1$ and shorter than 2 ns for $x=0.05$, suggesting an increase with increasing Rh concentration. Values of ${B}_{\mathrm{loc}}^{\mathrm{rms}}$ are on the order of 1 mT, consistent with the polarized neutron diffraction cross section.

Dimensionality is a critical factor in determining the properties of solids and is an apparent built-in character of the crystal structure. However, it can be an emergent and tunable property in geometrically frustrated spin systems. Here, we study the spin dynamics of the tetrahedral cluster antiferromagnet, pharmacosiderite, via muon spin resonance and neutron scattering. We find that the spin correlation exhibits a two-dimensional characteristic despite the isotropic connectivity of tetrahedral clusters made of spin 5/2 Fe3+ ions in the three-dimensional cubic crystal, which we ascribe to two-dimensionalisation by geometrical frustration based on spin wave calculations. Moreover, we suggest that even one-dimensionalisation occurs in the decoupled layers, generating low-energy and one-dimensional excitation modes, causing large spin fluctuation in the classical spin system. Pharmacosiderite facilitates studying the emergence of low-dimensionality and manipulating anisotropic responses arising from the dimensionality using an external magnetic field., 22 pages, 22 figures

Ultracold muonium (UCMu) is an important muonium (Mu) source for the generation of ultraslow muon beam for nanotechnological applications and for understanding hydrogen dynamics in materials. This study attempts to discover a new material for the generation of UCMu in vacuum. Muon spin rotation and relaxation measurements for n-type silicon (Si) between 7 K and 300 K were performed. The Mu rotation signal was evident at temperatures less than 200 K, indicating its presence. Moreover, the variation in relaxation rates of Mu formed deep inside and near the rear surface of Si provides hint of surface effect such as the generation of UCMu in vacuum. The yield of UCMu will be estimated via space–time resolution measurements in future research.

Molecular based spin-1/2 triangular lattice systems such as LiZn$_2$Mo$_3$O$_8$ have attracted research interest. Distortions, defects, and intersite disorder are suppressed in such molecular-based magnets, and intrinsic geometrical frustration gives rise to unconventional and unexpected ground states. Li$_2$$A$Mo$_3$O$_8$ ($A$ = In or Sc) is such a compound where spin-1/2 Mo$_3$O$_{13}$ clusters in place of Mo ions form the uniform triangular lattice. Their ground states are different according to the $A$ site. Li$_2$InMo$_3$O$_8$ undergoes conventional $120^\circ$ long-range magnetic order below $T_\text{N}=12$~K whereas isomorphic Li$_2$ScMo$_3$O$_8$ exhibits no long-range magnetic order down to 0.5~K. Here, we report exotic magnetisms in Li$_2$InMo$_3$O$_8$ and Li$_2$ScMo$_3$O$_8$ investigated by muon spin rotation ($\mu$SR) and inelastic neutron scattering (INS) spectroscopies using polycrystalline samples. Li$_2$InMo$_3$O$_8$ and Li$_2$ScMo$_3$O$_8$ show completely different behaviors observed in both $\mu$SR and INS measurements, representing their different ground states. Li$_2$InMo$_3$O$_8$ exhibits spin wave excitation which is quantitatively described by the nearest neighbor anisotropic Heisenberg model based on the $120^\circ$ spin structure. In contrast, Li$_2$ScMo$_3$O$_8$ undergoes short-range magnetic order below 4~K with quantum-spin-liquid-like magnetic fluctuations down to the base temperature. Origin of the different ground states is discussed in terms of anisotropies of crystal structures and magnetic interactions., Comment: Sci. Rep. 9, 1826 (2019)

${\mathrm{LiV}}_{2}{\mathrm{O}}_{4}$ spinel is known to exhibit heavy-fermion-like behavior below a characteristic temperature ${T}_{K}\ensuremath{\simeq}20$ K, while it preserves a paramagnetic state down to $T\ensuremath{\sim}{10}^{\ensuremath{-}2}$ K due to geometrical frustration. Here, it is shown that the dynamical spin susceptibility $\ensuremath{\chi}(\mathbf{q},\ensuremath{\omega})$ in ${\mathrm{LiV}}_{2}{\mathrm{O}}_{4}$ exhibits an anomalous duality which is modeled as a sum of itinerant (${\ensuremath{\chi}}_{\mathrm{F}}$) and local (${\ensuremath{\chi}}_{\mathrm{L}}$) components, and that the local spin dynamics inferred from ${\ensuremath{\chi}}_{\mathrm{L}}(\mathbf{q},\ensuremath{\omega})$ is qualitatively different from that expected from time-averaged bulk properties. The anomaly coexists with the marginal Fermi-liquid behavior inferred from the $\ensuremath{-}lnT$ dependence of the electronic specific heat over a wide temperature range below ${T}_{K}$. We argue that such unusual properties of ${\mathrm{LiV}}_{2}{\mathrm{O}}_{4}$ can be attributed to the putative metallic spin-liquid state emerging near the quantum critical point between spin-glass and Fermi-liquid states.

High precision measurements of the ground state hyperfine structure (HFS) of muonium is a stringent tool for testing bound-state quantum electrodynamics (QED) theory, determining fundamental constants of the muon magnetic moment and mass, and searches for new physics. Muonium is the most suitable system to test QED because both theoretical and experimental values can be precisely determined. Previous measurements were performed decades ago at LAMPF with uncertainties mostly dominated by statistical errors. At the J-PARC Muon Science Facility (MUSE), the MuSEUM collaboration is planning complementary measurements of muonium HFS both at zero and high magnetic field. The new high-intensity muon beam that will soon be available at H-Line will provide an opportunity to improve the precision of these measurements by one order of magnitude. An overview of the different aspects of these new muonium HFS measurements, the current status of the preparation for high-field measurements, and the latest results at zero field are presented.

We have investigated the nature of the short-range magnetic phase region of ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{IrO}}_{4}$ ($0\ensuremath{\le}x\ensuremath{\le}0.14$). This so-called spin-orbit Mott insulator's predicted transition to superconductivity with electron doping has remained elusive. Instead, short-range magnetism prevails over an extensive doping range up to the La solubility limit. We find evidence for quantum critical-like fluctuations of the short-range magnetic phase, in both a scaling relation of the ac susceptibility $\ensuremath{\chi}{T}^{\ensuremath{\alpha}}$ and the magnetization $M{T}^{\ensuremath{\alpha}\ensuremath{-}1}$ with $\ensuremath{\alpha}=0.4--0.5$, as well as in the time-field scaling ($t/{H}^{\ensuremath{\gamma}}$) of the $\ensuremath{\mu}\mathrm{SR}$ asymmetry function. Our combined study of ac susceptibility and $\ensuremath{\mu}\mathrm{SR}$, on the one hand, uncovers a two-stage weak symmetry breaking to spin freezing or proximate spin glass and to a subsequent spin glass within the short-range magnetic phase. On the other hand, we demonstrate that magnetic correlations become more static above $x=0.06$ in spite of an apparent dilution of magnetic moments, indicative of enhanced inhomogeneities in a higher La-doping regime.

Muon spin relaxation (μSR) measurements were performed to investigate the dynamics of polybutadiene (PB) reinforced by zinc diacrylate (ZDA). The spin relaxation rate was found to be almost independent of temperature below the glass transition temperature T g , while it increased monotonically with temperature above T g . In addition, the spin relaxation rate for PB including 14 vol% ZDA, which has a higher elastic modulus, was slower than that of PB without ZDA. Thus, μSR can clearly distinguish the dynamics of PB in these two samples, with the spin relaxations mainly caused by the so-called α-process (above T g ) and fast β-process (below T g ). To examine this scenario in detail, we performed μSR experiments as a function of longitudinal magnetic field (LF). The motional relaxation rate for PB was independent of temperature below T g , confirming that the spin relaxation is mainly induced by the fast β-process. The α-process above T g was also confirmed based on the Vogel–Fulcher-type temperature dependence, and the motional relaxation rate in the α-process was decreased by ZDA aggregates. These findings clearly show that μSR measurements provide useful information on the dynamics in complex polymer systems.

Passivation of BaSi2 films is essentially important for their use in solar cell applications. We study the effect of atomic H on photoresponsivity enhancement of 0.5-μm-thick BaSi2 films. A H supply duration of 15 min enhances the photoresponsivity of the BaSi2 film by a factor of 10. We also investigate the electronic states of H in BaSi2 via muon spin rotation. An implanted muon (Mu+) beam binds electrons to form muonium (Mu+e−). Their response to thermal activation shows that Mu+e− accompanies a shallow energy level of approximately 31 meV below the conduction band minimum (CBM), indicating that atomic H also serves as an electronically active donor impurity in BaSi2. This result is in good agreement with ab initio studies showing that a localized half-filled peak appears approximately 40 meV below the CBM if the first neighbors of the H atom are one Si atom and one Ba atom.

We explore the electrical and magnetic properties of a fractal assembly of Josephson junctions with transparent interfaces. For this purpose, we employ an $\mathrm{Mg}/\mathrm{MgO}/\mathrm{Mg}{\mathrm{B}}_{2}$ nanocomposite with \ensuremath{\sim}16 vol. % of $\mathrm{Mg}{\mathrm{B}}_{2}$ nanograins, which are distributed in a fractal manner in the normal matrix. Irrespective of the low volume fraction of $\mathrm{Mg}{\mathrm{B}}_{2}$ nanograins, the nanocomposite behaves as a bulk-like superconductor, i.e., zero resistivity, perfect diamagnetism, and strong vortex pinning. Thus, a global Josephson phase coherence is achieved in the nanocomposite. The lower (${H}_{\mathrm{c}1\mathrm{J}}$) and higher (${H}_{\mathrm{c}2\mathrm{J}}$) critical fields of the Josephson network are exceptionally high (${H}_{\mathrm{c}1\mathrm{J}}=96\phantom{\rule{0.16em}{0ex}}\mathrm{Oe}$ and ${H}_{\mathrm{c}2\mathrm{J}}=83.5\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$) as compared to those reported previously for granular superconductors. This will give an example of robust macroscopic superconducting coherence derived from long-range proximity coupling among fractally distributed superconducting nanograins through quantum interference of Andreev quasiparticles. Transverse-field muon spin rotation measurements reveal that the mean internal field in the superconducting mixed state increases with decreasing temperature below which the Josephson phase coherence sets in, opposite to the diamagnetic response observed in magnetization measurements. This unusual behavior implies a highly disordered and fluctuating nature of the Josephson vortices in the present superconducting nanocomposite.

Using an intense negative muon (μ^{-}) source, we have studied the internal magnetic fields in a powder sample of magnesium hydride (MgH_{2}). By extracting the signal from the μ^{-} captured on Mg nuclei, we found that the negative muon spin rotation and relaxation (μ^{-}SR) spectra clearly showed a Kubo-Toyabe-type relaxation, which indicates a random magnetic field at the Mg site. The field distribution width obtained is very consistent with the predicted value at the Mg site estimated by dipole field calculations, supporting our claim to have observed the nuclear magnetic fields of hydrogens in MgH_{2}. As is the case with μ^{+}SR, μ^{-}SR promises to soon be an indispensable tool for materials analyses.

We report a muon spin rotation/relaxation ($\ensuremath{\mu}\mathrm{SR}$) study of single-crystalline samples of the $\ensuremath{\alpha}\ensuremath{-}{\mathrm{RuCl}}_{3}$ honeycomb magnet, which is presumed to be a model compound for the Kitaev-Heisenberg interaction. It is inferred from magnetic susceptibility and specific-heat measurements that the present samples exhibit successive magnetic transitions at different critical temperatures ${T}_{\mathrm{N}}$ with decreasing temperature, eventually falling into the ${T}_{\mathrm{N}}=7$ K antiferromagnetic (7 K) phase that has been observed in only single-crystalline specimens with the least stacking fault. Via $\ensuremath{\mu}\mathrm{SR}$ measurements conducted under a zero external field, we show that such behavior originates from a phase separation induced by the honeycomb plane stacking fault, yielding multiple domains with different ${T}_{\mathrm{N}}$'s. We also perform $\ensuremath{\mu}\mathrm{SR}$ measurements under a transverse field in the paramagnetic phase to identify the muon site from the muon-Ru hyperfine parameters. Based on a comparison of the experimental and calculated internal fields at the muon site for the two possible spin structures inferred from neutron diffraction data, we suggest a modulated zigzag spin structure for the 7 K phase, with the amplitude of the ordered magnetic moment being significantly reduced from that expected for the orbital quenched spin-1/2 state.